Communication apparatus, communication system, and communication control method

ABSTRACT

An apparatus includes a first port coupled to a first apparatus through a ring line, a second port coupled to a second apparatus through a second line, the second apparatus disposed external to the ring line, the second line configured redundantly with a first line between the first apparatus and the second apparatus, a third port coupled to the first apparatus through a control line for a control signal concerning the first line, and circuitry that detects a fault in the control line, wherein the circuitry detects a fault in the second line, switches the second line from an active line to a standby line in accordance with the detection of the fault in the second line, and while the fault in the control line is detected, shuts down the first port in accordance with the detection of the fault in the second line.

CROSS-REFERENCE TO RELATED APPLICATION

This application is based upon and claims the benefit of priority of theprior Japanese Patent Application No. 2018-240286, filed on Dec. 21,2018, the entire contents of which are incorporated herein by reference.

FIELD

The embodiment discussed herein is related to a communication apparatus,a communication system, and a communication control method.

BACKGROUND

There is a communication system in which an access line for accessing aring network is made redundant by using a multi-chassis link aggregation(MC-LAG) technology. In a case where a fault occurs in the communicationbetween communication apparatuses coupled to a redundant access line, amain signal path is switched to a bypass for a location of a fault byusing, for example, an Ethernet (registered trademark; the same applieshereinafter) ring protection (ERP) function.

In the above instance, the monitoring control function of MC-LAG isadditionally used so that a control signal path is also switched to abypass for redundant system switchover for the communicationapparatuses. Therefore, the time required for switching a main signaland a control signal becomes longer than in a case where only the mainsignal path is switched. Additionally, a bandwidth of the control signalis obtained for an increased number of links in accordance with a pathlength increase from a pre-switch state. This reduces the bandwidth ofthe main signal.

For example, Japanese Laid-open Patent Publication No. 2015-211402 isdisclosed as a related art.

SUMMARY

According to an aspect of the embodiment, a communication apparatusincludes a first port that is coupled to a first communication apparatusthrough a ring line, a second port that is coupled to a secondcommunication apparatus through a second access line, the secondcommunication apparatus being disposed external to the ring line, thesecond access line being configured redundantly with a first access linebetween the first communication apparatus and the second communicationapparatus, a third port that is coupled to the first communicationapparatus through a control line for transmitting and receiving acontrol signal concerning the first access line, and circuitry thatdetects a fault in the control line, wherein the circuitry detects afault in the second access line, switches the second access line from anactive line to a standby line in accordance with the detection of thefault in the second access line, and while the fault in the control lineis detected, shuts down the first port in accordance with the detectionof the fault in the second access line.

The object and advantages of the invention will be realized and attainedby means of the elements and combinations particularly pointed out inthe claims.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory and arenot restrictive of the invention.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a configuration diagram illustrating an example of acommunication system;

FIG. 2 is a configuration diagram illustrating an example of acommunication apparatus;

FIG. 3 is a diagram illustrating an exemplary operation that isperformed by a communication system when a fault occurs in a controlline;

FIG. 4 is a diagram illustrating an exemplary operation that isperformed by a communication system when a fault occurs in a ring line;

FIG. 5 is a diagram illustrating an exemplary operation that isperformed by a communication system when a fault occurs in an accessline;

FIG. 6 is a diagram illustrating an exemplary operation that isperformed by a communication system when each fault occurs in an accessline and in a control line;

FIG. 7 is a diagram illustrating an exemplary operation that isperformed by a communication system in an event of a fault in a controlline and a defect in a ring apparatus;

FIG. 8 is a state transition diagram illustrating exemplary states of aring apparatus managed by a state control section;

FIG. 9 is a flowchart illustrating an example of a state control processperformed on a ring apparatus;

FIG. 10 is a flowchart illustrating an example of a process that isperformed in accordance with a state of a ring apparatus of an activesystem;

FIG. 11 is a flowchart illustrating an example of a process that isperformed in accordance with a state of a ring apparatus of a standbysystem;

FIG. 12 is a sequence diagram illustrating exemplary operations that areperformed by redundantly paired ring apparatuses when a fault in anaccess line is detected while a fault in a control line is detected;

FIG. 13 is a sequence diagram illustrating exemplary operations that areperformed by redundantly paired ring apparatuses when a defect isdetected while a fault in a control line is detected;

FIG. 14 is a configuration diagram illustrating an example of acommunication system in which a ring line between ring apparatuses isduplexed;

FIG. 15 is a diagram illustrating an exemplary operation that isperformed by a communication system when a fault occurs in one ringline;

FIG. 16 is a configuration diagram illustrating another example of aring apparatus;

FIG. 17 is a flowchart illustrating an example of a control processperformed on a main signal and a control signal; and

FIG. 18 is a state transition diagram illustrating exemplary states of aring apparatus managed by a state control section.

DESCRIPTION OF EMBODIMENT

Meanwhile, in a case where a communication line between communicationapparatuses is duplexed to use one communication line as an intra-portallink (IPL) for a control signal, even if a failure occurs in the IPLcommunication line or a failure occurs in the main signal, it may becontrolled so as not to affect the main signal. Only a main signal pathneeds to be switched even if a fault occurs in a main signalcommunication line. This suppresses an increase in the time required forswitchover and an increase in the bandwidth of the control signal.

However, when a fault occurs in the communication line for the IPL, itis difficult for the communication apparatuses to transmit and receivethe control signal to and from each other. Therefore, a redundant systemmay not be able to achieve switchover. Consequently, if a fault furtheroccurs, for example, in an access line of an active system, it might bepractically difficult to maintain main signal communication.

In view of the above circumstances, it is desirable to provide acommunication apparatus, a communication system, and a communicationcontrol method that are capable of maintaining main signal communicationeven if a fault occurs in a situation where the transmission of acontrol signal to a redundant partner communication apparatus isunachievable.

According to an aspect of an embodiment, it is possible to maintaincommunication even if a fault occurs in a situation where thetransmission of a control signal to a redundant partner communicationapparatus is unachievable.

FIG. 1 is a configuration diagram illustrating an example of acommunication system. The communication system includes terminalapparatuses 90 and 91 such as personal computers and smartphones, accessapparatuses 92 and 93, and ring apparatuses 1 a to 1 d forming a ringnetwork NW. For example, a layer 2 switch may be used as the accessapparatuses 92 and 93 and the ring apparatuses 1 a to 1 d. However, theaccess apparatuses 92 and 93 and the ring apparatuses 1 a to 1 d are notlimited to the layer 2 switch. A router or other relay apparatus may beused as access apparatuses 92 and 93 and the ring apparatuses 1 a to 1d. The ring apparatuses 1 a to 1 d are examples of a communicationapparatus. In the following example, however, operations of thecommunication apparatus 1 a are described.

The terminal apparatuses 90 and 91 are respectively coupled to theaccess apparatuses 92 and 93 through, for example, a wired local areanetwork (LAN) or a wireless LAN. The access apparatus 92 disposed towardthe terminal apparatus 90 is coupled to the ring apparatuses 1 a and 1 bthrough redundantly paired access lines La and Lb, respectively. Theaccess apparatus 93 disposed toward the terminal apparatus 91 is coupledto the ring apparatuses 1 c and 1 d through redundantly paired accesslines Lc and Ld, respectively.

The ring apparatuses 1 a to 1 d are coupled to a ring line Lr by using,for example, the ring network NW. The ring line Lr has an ERP function.The ring apparatuses 1 a to 1 d respectively include ports A1 to A4,ports B1 to B4, ports C1 to C4, and ports D1 to D4. The ports A1 to A4,ports B1 to B4, ports C1 to C4, and ports D1 to D4 each include, forexample, a transmitter for transmitting an optical signal and a receiverfor receiving an optical signal.

The port A2 of the ring apparatus 1 a is coupled to the access line La,and the port B2 of the ring apparatus 1 b is coupled to the access lineLb. The ports A2 and B2 are set so that the access lines La and Lbconfigure an MC-LAG for the access apparatus 92. In an initial state,the access line La is set as an active line, and the access line Lb isset as a standby line.

The port A3 of the ring apparatus 1 a is coupled to oppose the port B3of the ring apparatus 1 b through a control line Lf. The ports A3 and B3transmit and receive a control signal Sc concerning the access lines Laand Lb through the control line Lf. The control signal Sc includes, forexample, a fault notification for switching the access lines La and Lbfrom a standby line to an active line.

The ports A1 and A4 of the ring apparatus 1 a and the ports B1 and B4 ofthe ring apparatus 1 b are coupled to the ring line Lr. The port A1 andthe port B1 are intercoupled so as to oppose each other through the ringline Lr. The ports A1, A4, B1, and B4 transmit and receive, through thering line Lr, a main signal Sg including data transmitted by theterminal apparatuses 90 and 91.

The port C2 of the ring apparatus 1 c is coupled to the access line Lc,and the port D2 of the ring apparatus 1 d is coupled to the access lineLd. The ports C2 and D2 are set so that the access lines Lc and Ldconfigure the MC-LAG for the access apparatus 93. In the initial state,the access line Lc is set as an active line, and the access line Ld isset as a standby line. The pair of ring apparatuses 1 a and 1 b and thepair of ring apparatuses 1 c and 1 d are examples of the communicationsystem. In the following example, however, operations of the pair ofring apparatuses 1 a and 1 b are described.

The port C3 of the ring apparatus 1 c is coupled to oppose the port D3of the ring apparatus 1 d through a control line Le. As is the case withthe ports A3 and B3 and the ports C3 and D3 transmit and receive acontrol signal concerning the access lines Lc and Ld through the controlline Le.

The ports C1 and C4 of the ring apparatus 1 c and the ports D1 and D4 ofthe ring apparatus 1 d are coupled to the ring line Lr. The port C4 iscoupled to oppose the port B4 through the ring line Lr, and the port D4is coupled to oppose the port A4 through the ring line Lr. The port C1and the port D1 are intercoupled so as to oppose each other through thering line Lr. The ports C1, C4, D1, and D4 transmit and receive, throughthe ring line Lr, the main signal including data transmitted by theterminal apparatuses 90 and 91.

The terminal apparatuses 90 and 91 communicate with each other, forexample, through a path R. The path R runs through the access lines Laand Lc and through the ring line Lr between the ring apparatuses 1 a, 1b, and 1 c. A path between the port D1 of the ring apparatus 1 d and theport C1 of the ring apparatus 1 c is set as a blocking point BP in orderto suppress the main signal from continuously looping through the ringline Lr. Therefore, the ports C1 and D1 may not be able to transmit andreceive the main signal.

Configurations of the communication apparatuses 1 a to 1 d will now bedescribed. The following example deals with the communication apparatus1 a. However, the other communication apparatuses 1 b to 1 d haveconfiguration similar to that of the communication apparatus 1 a.

FIG. 2 is a configuration diagram illustrating an example of thecommunication apparatus 1 a. The communication apparatus 1 a includes acentral processing unit (CPU) 10, a read only memory (ROM) 11, a randomaccess memory (RAM) 12, a hardware interface section (HW-IF) 13, aswitch (SW) device 14, a switch memory 15, and ports A1 to A4. The CPU10 is coupled to the ROM 11, the RAM 12, and the HW-IF 13 through a bus19 so that signals may be inputted and outputted between them.

The ROM 11 stores programs that drive the CPU 10. The programs include,for example, software for executing a communication control methodaccording to the embodiment. The RAM 12 functions as a working memoryfor the CPU 10.

The HW-IF 13 processes the communication between the CPU 10, the switchdevice 14, and the ports A1 to A4. The HW-IF 13 is a circuit includinghardware such as a field programmable gate array (FPGA) or anapplication specified integrated circuit (ASIC).

The ports A1 to A4 are, for example, LAN ports, and each include anoptical transceiver (not illustrated) for transmitting and receiving anEthernet signal. The Ethernet signal includes, in addition to the mainsignal Sg, monitoring control data concerning the path switchover of themain signal Sg for ERP.

The port A1, which is an example of a first port, is coupled to the portB1 of the communication apparatus 1 b through the ring line Lr. Thecommunication apparatus 1 b is an example of a first communicationapparatus.

The port A2, which is an example of a second port, is coupled to theaccess apparatus 92 through the access line La. As mentioned earlier,the access line La is configured redundantly with the access line Lbbetween the access apparatus 92 and the ring apparatus 1 b. The accessapparatus 92 is an example of a second communication apparatus disposedexternal to the ring line Lr, and the access lines La and Lb are anexample of a second access line and an example of a first access line,respectively. The port B2 of the ring apparatus 1 b is an example of anaccess port that is coupled to the access apparatus 92 through theaccess line Lb.

The port A3, which is an example of a third port, is coupled to the portB3 of the ring apparatus 1 b through the control line Lf. The controlline Lf is configured, for example, as an IPL. Therefore, the mainsignal Sg does not flow through the control line Lf, and only thecontrol signal Sc flows through the control line Lf.

The port A4, which is an example of a fourth port, is coupled to theport D4 of the ring apparatus 1 d through the ring line Lr. When thepath R of the main signal Sg runs through the ports A4 and D4 and portsA4 and D4 transmit and receive the main signal Sg through the ring lineLr. The communication apparatus 1 d is an example of a thirdcommunication apparatus.

The switch device 14 is coupled to the ports A1 to A4. The switch device14 includes monitoring circuits 141 to 144 that monitor thecommunication state of each of the ports A1 to A4. For example, themonitoring circuits 141 to 144 output an alarm upon detection of a biterror rate of the communication along the access line La and the ringline Lr or upon detection of an interrupted optical input or output.

The switch device 14 also mediates the exchange of main signal Sgbetween the ports A1, A2, and A4. When the main signal Sg is transferredalong the route R illustrated in FIG. 1, the switch device 14 transfersthe main signal Sg between the ports A1 and A2. The switch device 14 isa circuit including hardware such as an FPGA or an ASIC.

The switch device 14 is coupled to the switch memory 15 that stores anaddress learning table (TBL) 151. The switch device 14 learns theaddress of an Ethernet signal from the main signal Sg and stores thelearned address in the address learning table 151. For example, thedestination address of the main signal Sg transferred from the ports A1,A2, and A4 is registered in the address learning table 151.

The switch device 14 transfers the main signal Sg between the ports A1,A2, and A4 in accordance with the address learning table 151. When theCPU 10 uses an ERP function to issue an instruction for switching thepath R, the switch device 14 initializes the contents of the addresslearning table 151 and relearns addresses.

The switch device 14 transmits and receives the control signal Scthrough the port A3. The switch device 14 outputs the received controlsignal Sc to the CPU 10 through the HW-IF 13. Meanwhile, the controlsignal Sc to be transmitted is inputted from the CPU 10 to the switchdevice 14 through the HW-IF 13.

Upon reading a program from the ROM 11, the CPU 10 forms, as functions,a state control section 100, a defect detection section 101, an MCLAGcontrol section 102, an ERP control section 103, and fault detectionsections 104 to 107.

The fault detection section 104, which is an example of a firstdetection section, detects a fault in the control line Lf by collectingalarms from the monitoring circuit 143. The fault detection section 104detects a fault in the control line Lf when, for example, the controlsignal Sc is asynchronous with respect to the redundant partner ringapparatus 1 b. The fault in the control line Lf may be, for example, abreak in the optical fiber of the control line Lr or a defect of theport A3 or B3.

The fault detection section 105, which is an example of a seconddetection section, detects a fault in the access line La by collectingalarms from the monitoring circuit 142. The fault in the access line Lamay be, for example, a break in the optical fiber of the access line Laor a defect of the port A4. The fault detection section 105 of the ringapparatus 1 b is an example of a third detection section that detects afault in the access line Lb.

The defect detection section 101 detects a defect in the ring apparatus1 a. The defect detection section 101 detects a defect when, forexample, a periodic inspection signal from a monitoring target circuitis discontinued.

The fault detection section 106, which is an example of a ring faultdetection section, detects a fault in the ring line Lr toward the portA1 by collecting alarms from the monitoring circuit 141. The fault inthe ring line Lr toward the port A1 may be, for example, a break in theoptical fiber between the ports A1 and B1 or a defect of the port A1etc.

The fault detection section 107 detects a fault in the ring line Lrtoward the port A4 by collecting alarms from the monitoring circuit 144.The fault in the ring line Lr toward the port A4 may be, for example, abreak in the optical fiber between the ports A4 and D4 or a defect ofthe port A4 etc.

The state control section 100 coordinates with the MCLAG control section102 and the ERP control section 103 to control the state of the ringapparatus 1 a in accordance with given conditions. The state controlsection 100 controls the ports A1 to A4 in accordance with the state ofthe ring apparatus 1 a.

Based on the results of detection by the fault detection sections 104and 105 and the defect detection section 101, the MCLAG control section102 provides redundant system switching control of the access lines Laand Lb redundantly configured by MC-LAG. When no fault is detected inthe control line Lf by the fault detection section 104, the MCLAGcontrol section 102 communicates with the ring apparatus 1 b through thecontrol line Lf.

For example, when no fault is detected in the control line Lf, the MCLAGcontrol section 102 generates, depending on the detection of a fault inthe access line La by the fault detection section 105, the controlsignal Sc including a fault notification concerning the fault in theaccess line La, and outputs the generated control signal Sc to theswitch device 14 through the HW-IF 13. Consequently, the faultnotification concerning the fault in the access line La is transmittedfrom the port A3 to the redundant partner ring apparatus 1 b through thecontrol line Lf.

The port B3 of the ring apparatus 1 b receives signals on the controlline Lf, including a fault notification. The switch device 14 outputs afault notification inputted from the port B3 to the CPU 10 through theHW-IF 13. Based on the fault notification, the MCLAG control section 102of the ring apparatus 1 b switches the access line Lb from a standbyline to an active line. In this instance, the MCLAG control section 102exercises control to switch the port B2 of the access line Lb from anon-communicative state to a communicative state. The MCLAG controlsection 102 of the ring apparatus 1 b is an example of a secondswitching section.

As described above, the MCLAG control section 102 of the ring apparatus1 a transmits, to the ring apparatus 1 b at a neighboring node throughthe control line Lf, the control signal Sc for switching the access lineLb from a standby line to an active line. Therefore, in the path R ofthe main signal Sg, the faulty access line La is replaced by thenonfaulty access line Lb. Consequently, the main signal Sg iscommunicated continuously even after the occurrence of a fault. TheMCLAG control section 102 is an example of a switching section and of afirst switching section.

Without regard to the presence of a fault in the control line Lf, theMCLAG control section 102 of the ring apparatus 1 a switches the accessline La from an active line to a standby line in accordance with thedetection of a fault in the access line La by the fault detectionsection 105. In this instance, the MCLAG control section 102 exercisescontrol to switch the port A2 of the access line La from a communicativestate to a non-communicative state.

As described above, when a fault occurs in the access line La, the ringapparatus 1 a switches from an active system to a standby system.

However, if a fault occurs in the access line La while the control lineLf is faulty, the ring apparatus 1 a may not be able to transmit a faultnotification to the redundant partner ring apparatus 1 b through thecontrol line Lf.

Accordingly, while a fault in the control line Lf is detected, the statecontrol section 100 shuts down the port A1 in accordance with thedetection of a fault in the access line La. The state control section100 exercises control to switch the port A1 from a communicative stateto a non-communicative state, for example, through the HW-IF 13. Thiscauses the port A1 to stop the transmission and reception of the mainsignal Sg.

In the redundant partner ring apparatus 1 b, the monitoring circuit 141detects the shutdown of the port A1 when, for example, an optical inputfrom the port A1 is discontinued. The fault detection section 106acquires an alarm indicative of the shutdown from the monitoring circuit141, and notifies the ERP control section 103 of the shutdown. Themonitoring circuit 141 is an example of a fourth detection section thatdetects a shutdown.

When no fault is detected in the access line Lb by the fault detectionsection 105 while a fault in the control line Lf is detected, the statecontrol section 100 of the ring apparatus 1 b coordinates with the ERPcontrol section 103 to switch the access line Lb from a standby line toan active line in accordance with the detection of a shutdown of theport A1. This enables the access apparatus 92 to communicate with thering apparatus 1 b through the access line Lb instead of the faultyaccess line La.

Consequently, even if a fault occurs in a case where the transmission ofthe control signal Sc to the redundant partner ring apparatus 1 b isunachievable, the ring apparatus 1 a is able to maintain communication.The state control section 100 is an example of a control section thatshuts down the port A1.

The MCLAG control section 102 switches the access line La from an activeline to a standby line even in a case where a defect is detected by thedefect detection section 101. In this instance, the state controlsection 100 shuts down all the ports A1 to A4 so that the transmissionof the main signal Sg and control signal Sc in a defect state does notallow the defect to affect the other ring apparatuses 1 a to 1 d andaccess apparatus 92. The MCLAG control section 102 of the ring apparatus1 b switches the access line Lb from a standby line to an active line inaccordance with the detection of a shutdown of the port A1.

Consequently, even when a defect occurs in one ring apparatus 1 a, theother ring apparatus 1 b is able to maintain communication. However, ifa defect occurs while the control line Lf is faulty, the ring apparatus1 a may not be able to transmit the control signal Sc to the redundantpartner ring apparatus 1 b.

Accordingly, the state control section 100 of the ring apparatus 1 ashuts down all the ports A1 to A4 in accordance with the detection of adefect by the defect detection section 101. In this instance, the statecontrol section 100 exercises control to switch the ports A1 to A4 froma communicative state to a non-communicative state through the HW-IF 13.

Consequently, the MCLAG control section 102 of the partner ringapparatus 1 b switches the access line Lb from a standby line to anactive line in accordance with the detection of a shutdown of the portA1. Therefore, even when a defect occurs, one redundant ring apparatus 1a is able to let the other ring apparatus 1 b maintain communication.

The ring apparatus 1 a shuts down all the ports A1 to A4. Thissuppresses a defect from affecting the access apparatus 92 and the otherring apparatuses 1 b to 1 d when, for example, an abnormal main signalSg or control signal Sc generated due to the defect is transmitted.

The ERP control section 103 controls the communication of the mainsignal Sg through the ring line Lr in accordance with the results ofdetection by the fault detection sections 106 and 107 and the defectdetection section 101. The ERP control section 103 switches the path Rof the main signal Sg in accordance with the detection of a fault in thering line Lr by the fault detection sections 106 and 107. The ERPcontrol section 103 sets the blocking point BP in accordance with theswitched path R. Therefore, the communication within the ring line Lr ismaintained even if a fault occurs in the ring line Lr between the ringapparatuses 1 a and 1 b or between the ring apparatuses 1 a and 1 c.

If there is no fault in at least either one of the ring line Lr betweenthe ring apparatus 1 a and the ring apparatus 1 b and the ring line Lrbetween the ring apparatus 1 a and the communication apparatus 1 d in acase where neither a fault in the access line La nor a defect in thering apparatus 1 a is detected, the MCLAG control section 102 maintainsthe access line Lc as an active line.

The main signal Sg does not flow through the control line Lf. Therefore,even while the control line Lf is faulty, the ring apparatus 1 a is ableto maintain the communication of the main signal Sg from at least eitherone of the ports A1 and A4 through the ring line Lr as far as no faultoccurs in the access line La and no defect occurs in the ring apparatus1 a. Consequently, the MCLAG control section 102 does not switch theaccess line La from an active line to a standby line, and thus saves thetime required for such switching.

Exemplary operations performed in the event of a fault or a defect inthe ring apparatus 1 a will now be described. The following exemplaryoperations are described on the assumption that the initial state of thecommunication system is as illustrated in FIG. 1.

(In the Event of a Fault in the Control Line Lf)

FIG. 3 is a diagram illustrating an exemplary operation that isperformed by the communication system when a fault occurs in the controlline Lf. Elements common to FIGS. 1 and 3 are designated by the samereference symbols and will not be redundantly described.

A cross mark (x) indicates an exemplary location of a fault. The sameapplies to the subsequent exemplary operations. In the present example,a fault has occurred in the control line Lf, but no fault has occurredin the access line La and in the ring line Lr, and no defect hasoccurred in the ring apparatus 1 a.

Accordingly, the MCLAG control section 102 of the ring apparatus 1 amaintains the access line La as an active line. This saves the timerequired for switching the access line La to a standby line. Even when afault occurs in the control line Lf, the ring apparatuses 1 a and 1 bstop the transmission and reception of the control signal Sc withoutsetting a path of the control signal Sc for the ring line Lr in such amanner as to bypass the fault. This suppresses the fault in the controlline Lf from affecting the main signal Sg in the ring line Lr.

(In the Event of a Fault in the Ring Line Lr)

FIG. 4 is a diagram illustrating an exemplary operation that isperformed by the communication system when a fault occurs in the ringline Lr. Elements common to FIGS. 1 and 4 are designated by the samereference symbols and will not be redundantly described.

In the present example, a fault has occurred in the ring line Lr betweenthe ports A1 and B1, but no fault has occurred in the access line La andin the ring line Lr between the ports A4 and D4, and no defect hasoccurred in the ring apparatus 1 a. Accordingly, the MCLAG controlsection 102 of the ring apparatus 1 a maintains the access line La as anactive line.

When the fault detection section 106 detects a fault in theabove-mentioned section of the ring line Lr, the ERP control section 103of the ring apparatuses 1 a and 1 b notifies each of the other ringapparatuses 1 d and 1 c of the detected fault through the ports A4 andB4. The ERP control section 103 of the respective ring apparatuses 1 ato 1 d switches the path R so as to bypass the fault. In this instance,the ERP control section 103 of the respective ring apparatuses 1 a to 1d sets the blocking point BP In a section where the fault of the ringline Lr has occurred. Therefore, the switched path R runs through theaccess apparatuses 92 and 93 and the ring apparatuses 1 a and 1 c.

As described above, since no fault has occurred in the ring line Lrbetween the ports A4 and D4, the ring apparatus 1 a is able tocontinuously communicate the main signal Sg by setting the path Rrunning through such a nonfaulty section of the ring line Lr withoutswitching the access line La to a standby line. This suppresses thefault in the ring line Lr from affecting the control line Lf.

(In the Event of a Fault in the Access Line La)

FIG. 5 is a diagram illustrating an exemplary operation that isperformed by the communication system when a fault occurs in the accessline La. Elements common to FIGS. 1 and 5 are designated by the samereference symbols and will not be redundantly described.

In the present example, a fault has occurred in the access line Latoward the ring apparatus 1 a, but no fault has occurred in the controlline Lf, and no defect has occurred in the ring apparatus 1 a.

Accordingly, the MCLAG control section 102 of the ring apparatus 1 aswitches the access line La from an active line to a standby line, andtransmits a fault notification concerning the fault in the access lineLa to the redundant partner ring apparatus 1 b through the control lineLf. In response to the fault notification, the MCLAG control section 102of the ring apparatus 1 b switches the access line Lb from a standbyline to an active line.

The ERP control section 103 of the ring apparatuses 1 a and 1 bcoordinates with the MCLAG control section 102 to switch the path R ofthe main signal Sg in accordance with the redundant system switching ofthe access lines La and Lb. Consequently, the communication of the mainsignal Sg is maintained even after the occurrence of the fault in theaccess line La.

(In the Event of a Fault in the Access Line La and in the Control LineLf)

FIG. 6 is a diagram illustrating an exemplary operation that isperformed by the communication system when a fault occurs in the accessline La and in the control line Lf. Elements common to FIGS. 1 and 6 aredesignated by the same reference symbols and will not be redundantlydescribed.

The present example deals with a case where a fault has occurred in theaccess line La in a state Illustrated in FIG. 3. In accordance with thedetection of a fault in the access line La, the MCLAG control section102 of the ring apparatus 1 a switches the access line La from an activeline to a standby line. However, due to a fault in the control line Lf,the ring apparatus 1 a may not be able to transmit, to the redundantpartner ring apparatus 1 b, the control signal Sc including a faultnotification concerning the fault in the access line La.

Accordingly, while the fault in the control line Lf is detected, thestate control section 100 of the ring apparatus 1 a shuts down the portA1 in accordance with the detection of the fault in the access line La.

When no fault is detected in the access line Lb by the fault detectionsection 105 while the fault in the control line Lf is detected, theMCLAG control section 102 of the redundant partner ring apparatus 1 bswitches the access line Lb from a standby line to an active line inaccordance with the detection of the shutdown of the port A1. Forexample, the MCLAG control section 102 switches the access line Lb froma standby line to an active line when the ring apparatus 1 b is not thecause of the shutdown.

Accordingly, even when the transmission of the control signal Sc to theredundant partner ring apparatus 1 b is unachievable, the ring apparatus1 a is able to issue a fault notification due to the shutdown of theport A1. This enables the ring apparatus 1 b to maintain communication.In the present example, the ERP control section 103 of each of the ringapparatuses 1 a and 1 b switches the blocking point BP and the path R ofthe main signal Sg, as is the case with the exemplary operationillustrated in FIG. 5.

As regards the ring apparatuses 1 a and 1 b, the active system and thestandby system interchange due to the above operation. This suppressesboth of the ring apparatuses 1 a and 1 b from becoming active.Consequently, this suppresses the occurrence of a loop in the mainsignal Sg transmitted through the access apparatus 92 and the ringapparatuses 1 a and 1 b.

(In the Event of a Fault in the Control Line Lf and a Defect in the RingApparatus 1 a)

FIG. 7 is a diagram illustrating an exemplary operation that isperformed by the communication system when a fault occurs in the controlline Lf and a defect occurs in the ring apparatus 1 a. Elements commonto FIGS. 1 and 7 are designated by the same reference symbols and willnot be redundantly described.

The present example deals with a case where a defect has occurred in thering apparatus 1 a in a state Illustrated in FIG. 3. Due to a localdefect in addition to a fault in the control line Lf, the ring apparatus1 a may not be able to notify the redundant partner ring apparatus 1 bof the defect.

The state control section 100 of the ring apparatus 1 a shuts down theports A1 to A4 in accordance with the detection of the defect. Theredundant partner ring apparatus 1 b switches the access line Lb from astandby line to an active line in accordance with the detection of theshutdown of the port A1, as is the case with the foregoing exemplaryoperation, and is thus able to maintain communication.

The ring apparatus 1 a shuts down all the ports A1 to A4. Therefore, thetransmission of an abnormal main signal Sg and control signal Sc, whichmay be generated due to a defect, is suppressed. This suppresses adefect in the ring apparatus 1 a from affecting the other ringapparatuses 1 b to 1 d.

(Processing in the CPU 10)

The processing performed by the CPU 10 will now be described.

FIG. 8 is a state transition diagram illustrating exemplary states ofthe ring apparatuses 1 a to 1 d managed by the state control section100. Defined states of the ring apparatuses 1 a to 1 d are, for example,a normal state, an asynchronous state, an access line fault state, and adefect state. Arrows connecting the individual states are marked withevents indicative of transition conditions.

The normal state is a regular state where neither a fault nor a defecthas occurred in the ring apparatuses 1 a to 1 d. The asynchronous stateis a state where the control signal Sc is not synchronized between thering apparatuses 1 a and 1 b or between the ring apparatuses 1 c and 1 ddue to a fault in the control lines Lf and Le.

The access line fault state is a state where the access lines La to Ldare faulty. The defect state is a state where the ring apparatuses 1 ato 1 d are defective.

In the normal state, the state control section 100 transitions the ringapparatuses 1 a to 1 d into the asynchronous state in accordance withthe occurrence of a fault in the control lines Lf and Le (see “CONTROLLINE FAULT OCCURRENCE”). Meanwhile, in the asynchronous state, the statecontrol section 100 transitions the ring apparatuses 1 a to 1 d into theregular state in accordance with the fault recovery of the control linesLf and Le (see “CONTROL LINE FAULT RECOVERY”).

In the normal state, the state control section 100 transitions the ringapparatuses 1 a to 1 d into the access line fault state in accordancewith the occurrence of a fault in the access lines La to Ld (see “ACCESSLINE FAULT OCCURRENCE”). Meanwhile, in the access line fault state, thestate control section 100 transitions the ring apparatuses 1 a to 1 dinto the regular state in accordance with the fault recovery of theaccess lines La to Ld (see “ACCESS LINE FAULT RECOVERY”).

In the asynchronous state, the state control section 100 transitions thering apparatuses 1 a to 1 d into the access line fault state inaccordance with the occurrence of a fault in the access lines La to Ld(see “ACCESS LINE FAULT OCCURRENCE”). Meanwhile, in the access linefault state, the state control section 100 transitions the ringapparatuses 1 a to 1 d into the asynchronous state in accordance withthe occurrence of a fault in the control lines Lf and Le (see “CONTROLLINE FAULT OCCURRENCE”) and with the fault recovery of the access linesLa to Ld (see “ACCESS LINE FAULT RECOVERY”).

In the normal state, in the access line fault state, and in theasynchronous state, the state control section 100 transitions the ringapparatuses 1 a to 1 d into an apparatus defect state in accordance withthe detection of a defect in the ring apparatuses 1 a to 1 d. The ringapparatuses 1 a to 1 d do not achieve recovery from a defect. Therefore,when the ring apparatuses 1 a to 1 d transition into the apparatusdefect state, the state control section 100 stops controlling statetransitions. In this instance, defective ring apparatuses 1 a to 1 d arereplaced by new ring apparatuses.

FIG. 9 is a flowchart illustrating an example of a state control processperformed on the ring apparatuses 1 a to 1 d. The ERP control section103 and the MCLAG control section 102 start conducting the main signalSg and the control signal Sc, respectively (step St1). In this instance,the ERP control section 103 and the MCLAG control section 102 performvarious setups, for example, on the ports A1 to A4 and the switch device14.

Next, the ERP control section 103 causes the fault detection sections106 and 107 to determine whether or not a fault is detected in the ringline Lr (step St2). If no fault is detected in the ring line Lr (“NO” atstep St2), later-described steps St5 and beyond are performed.

If a fault is detected in the ring line Lr (“YES” at step St2), the ERPcontrol section 103 changes the blocking point BP in the ring line Lr(step St3). In this instance, the ERP control section 103 exercisescontrol to place the ports A1 to A4, B1 to B4, C1 to C4, and D1 to D4corresponding to the blocking point BP in a non-communicative state.

Next, the ERP control section 103 switches the path R of the main signalSg in the ring line Lr (step St4). In this instance, the ERP controlsection 103 initializes the address learning table 151 and then causesthe switch device 14 to learn addresses.

Next, the state control section 100 checks the results of fault anddefect detection by the fault detection sections 104 and 105 and thedefect detection section 101 in order to determine whether or not anevent indicative of a state transition condition has occurred (stepSt5). If the event has not occurred (“NO” at step St5), steps St2 andbeyond are performed again.

If the event has occurred (“YES” at step St5), the state control section100 transitions the states of the ring apparatuses 1 a to 1 d inaccordance with the event as illustrated in FIG. 8 (step St6). Next, thestate control section 100 determines whether or not the ring apparatuses1 a to 1 d are in the defect state (step St7).

If the ring apparatuses 1 a to 1 d are not in the defect state (“NO” atstep St7), steps St2 and beyond are performed again. If the ringapparatuses 1 a to 1 d are in the defect state (“YES” at step St7), thestate control process terminates. The state control process is performedin the above-described manner.

FIG. 10 is a flowchart illustrating an example of a process that isperformed in accordance with the states of the ring apparatuses 1 a and1 c of the active system. The process is repeatedly performed inparallel with the state control process performed on the ringapparatuses 1 a and 1 c of the active system.

The state control section 100 determines whether or not the ringapparatuses 1 a and 1 c are in the defect state (step St11). If the ringapparatuses 1 a and 1 c are in the defect state (“YES” at step St11),the MCLAG control section 102 switches the access lines La and Lc froman active line to a standby line in accordance with an Instruction fromthe state control section 100 (step St12). Next, the state controlsection 100 shuts down all the ports A1 to A4 and C1 to C4 (step St13).

As described above, the MCLAG control section 102 switches the accesslines La and Lc from an active line to a standby line in accordance withthe detection of a defect, and the state control section 100 shuts downthe ports A1 to A4 and C1 to C4 in accordance with the detection of adefect. This permits the ring apparatuses 1 a and 1 b to switch theaccess lines Lb and Ld of the redundant partner ring apparatuses 1 b and1 d from a standby line to an active line, and suppresses the defectfrom affecting the other ring apparatuses 1 a to 1 d and accessapparatuses 92 and 93.

Meanwhile, if the ring apparatuses 1 a and 1 c are not in the defectstate (“NO” at step St11), the state control section 100 determineswhether or not the ring apparatuses 1 a and 1 c are in the asynchronousstate (step St14). If the ring apparatuses 1 a and 1 c are in theasynchronous state (“YES” at step St14), the state control section 100causes the fault detection section 105 to determine whether or not afault is detected in the access lines La and Lc (step St15).

If a fault is detected in the access lines La and Lc (“YES” at stepSt15), the MCLAG control section 102 switches the access lines La and Lcfrom an active line to a standby line in accordance with an instructionfrom the state control section 100 (step St16). Next, the state controlsection 100 shuts down the ports A1 and C1 of the ring line Lr (stepSt17). Meanwhile, if no fault is detected in the access lines La and Lc(“NO” at step St15), the process terminates.

As described above, the MCLAG control section 102 switches the accesslines La and Lc from an active line to a standby line in accordance withthe detection of a fault in the access lines La and Lc, and the statecontrol section 100 shuts down the port A1 in accordance with thedetection of a fault in the access lines La and Lc while a fault in thecontrol line Lf is detected. This permits the ring apparatuses 1 a and 1b to switch the access lines Lb and Ld of the redundant partner ringapparatuses 1 b and 1 d from a standby line to an active line.

Meanwhile, if the ring apparatuses 1 a and 1 c are not in theasynchronous state (“NO” at step St14), the state control section 100determines whether or not the ring apparatuses 1 a and 1 c are in theregular state (step St18). If the ring apparatuses 1 a and 1 c are notin the regular state (“NO” at step St18), the process terminates. If thering apparatuses 1 a and 1 c are in the regular state (“YES” at stepSt18), the state control section 100 causes the fault detection section105 to determine whether or not a fault is detected in the access linesLa and Lc (step St19).

If a fault is detected in the access lines La and Lc (“YES” at stepSt19), the MCLAG control section 102 switches the access lines La and Lcfrom an active line to a standby line in accordance with an instructionfrom the state control section 100 (step St20). Next, the state controlsection 100 transmits a fault notification concerning a fault in theaccess lines La and Lc to the redundant partner ring apparatuses 1 b and1 d (step St21). Meanwhile, if no fault is detected in the access linesLa and Lc (“NO” at step St19), the process terminates.

As described above, when no fault is detected in the control lines Lfand Le, the MCLAG control section 102 transmits a fault notification tothe ring apparatuses 1 b and 1 d of the standby system through thecontrol lines Lf and Le in accordance with the detection of a fault inthe access lines La and Lc. This permits the ring apparatuses 1 a and 1b to switch the access lines Lb and Ld of the redundant partner ringapparatuses 1 b and 1 d from a standby line to an active line. In theabove-described manner, the ring apparatuses 1 a and 1 c of the activesystem perform the process in accordance with the current state.

FIG. 11 is a flowchart illustrating an example of a process that isperformed in accordance with the state of the ring apparatuses 1 b and 1d of the standby system. The process is repeatedly performed in parallelwith the state control process performed on the ring apparatuses 1 b and1 d of the standby system.

The state control section 100 determines whether or not the ringapparatuses 1 b and 1 d are in the asynchronous state (step St31). Ifthe ring apparatuses 1 b and 1 d are in the asynchronous state (“YES” atstep St31), the state control section 100 determines whether or not theshutdown of the ports A1 and C1 of the redundant partner ringapparatuses 1 a and 1 c is detected by the monitoring circuit 141 (stepSt32). The monitoring circuit 141 detects the shutdown when, forexample, an optical input from the ports A1 and C1 is discontinued.

If the shutdown is not detected (“NO” at step St32), the processterminates. Meanwhile, if the shutdown is detected (“YES” at step St32),the state control section 100 determines whether or not the monitoringcircuits 141 to 144 have outputted an alarm that may cause the shutdown(step St33). If the alarm is outputted (“YES” at step St33), the processterminates.

If the alarm that may cause the shutdown is not outputted (“NO” at stepSt33), the MCLAG control section 102 determines whether or not a faultis detected in the access lines Lb and Ld by the fault detection section105 (step St34). If a fault is detected in the access lines Lb and Ld(“YES” at step St34), the process terminates.

If no fault is detected in the access lines Lb and Ld (“NO” at stepSt34), the MCLAG control section 102 switches the access lines Lb and Ldfrom a standby line to an active line (step St35). Next, the ERP controlsection 103 switches the path R of the main signal Sg so that the path Rruns through the access lines Lb and Ld (step St36). Upon completion ofstep St36, the process terminates.

As described above, when no fault is detected in the access lines La andLc while a fault in the control lines Lf and Le is detected, the MCLAGcontrol section 102 switches the access lines Lb and Ld from a standbyline to an active line in accordance with the detection of the shutdownof the ports A1 and C1 of the redundant partner ring apparatuses 1 a and1 c. Therefore, the ring apparatuses 1 b and 1 d are able to maintainthe communication of the main signal Sg.

Meanwhile, if the ring apparatuses 1 b and 1 d are not in theasynchronous state (“NO” at step St31), the state control section 100determines whether or not the ring apparatuses 1 b and 1 d are in theregular state (step St37). If the ring apparatuses 1 b and 1 d are notin the regular state (“NO” at step St37), the process terminates.

If the ring apparatuses 1 b and 1 d are in the regular state (“YES” atstep St37), the MCLAG control section 102 determines whether or not afault notification concerning a fault in the access lines La and Lc isreceived from the redundant partner ring apparatuses 1 a and 1 c (stepSt38). If the fault notification is not received (“NO” at step St38),the process terminates. Meanwhile, if the fault notification is received(“YES” at step St38), steps St35 and St36 are performed. Upon completionof steps St35 and St36, the process terminates. In the above-describedmanner, the ring apparatuses 1 b and 1 d of the standby system performthe process in accordance with the current state.

FIG. 12 is a sequence diagram Illustrating exemplary operations that areperformed by redundantly paired ring apparatuses 1 a and 1 b when afault in the access line La is detected while a fault in the controlline Lf is detected. The present example deals with the operationsperformed by the pair of ring apparatuses 1 a and 1 b. However, theother pair of ring apparatuses 1 c and 1 d performs similar operationsas the pair of ring apparatuses 1 a and 1 b. At the beginning of asequence illustrated in FIG. 12, the ring apparatuses 1 a and 1 b areboth in the normal state, the ring apparatus 1 a operates as an activesystem, and the ring apparatus 1 b operates as a standby system.

Upon detecting a fault in the control line Lf (symbol SQ1 a), the ringapparatus 1 a of the active system switches from the normal state to theasynchronous state. Upon detecting a fault in the control line Lf(symbol SQ1 b), the ring apparatus 1 b of the standby system switchesfrom the normal state to the asynchronous state.

Subsequently, upon detecting a fault in the access line La in theasynchronous state (symbol SQ2), the ring apparatus 1 a of the activesystem shuts down the port A1 (symbol SQ3). The ring apparatus 1 a thenswitches from the active system to the standby system, and transitionsinto the access line fault state.

Upon detecting the shutdown of the port A1 in the asynchronous state(symbol SQ4), the ring apparatus 1 b of the standby system switches fromthe standby system to the active system. As described above, when theport A1 shuts down in a case where a fault in the access line La isdetected while a fault in the control line Lf is detected, the ringapparatuses 1 a and 1 b switch between the active system and the standbysystem.

FIG. 13 is a sequence diagram illustrating exemplary operations that areperformed by redundantly paired ring apparatuses 1 a and 1 b when adefect is detected while a fault in the control line Lf is detected.Operations common to FIGS. 12 and 13 are designated by the samereference symbols and will not be redundantly described.

The present example deals with the operations performed by the pair ofring apparatuses 1 a and 1 b. However, the other pair of ringapparatuses 1 c and 1 d performs similar operations as the pair of ringapparatuses 1 a and 1 b. At the beginning of a sequence Illustrated inFIG. 13, the ring apparatuses 1 a and 1 b are both in the normal state,the ring apparatus 1 a operates as an active system, and the ringapparatus 1 b operates as a standby system.

Upon detecting a defect (symbol SQ5), the ring apparatus 1 a of theactive system switches to the ring apparatus of the standby system andtransitions into the defect state. The ring apparatus 1 a then shutsdown the ports A1 to A4 (symbol SQ6).

Upon detecting the shutdown of the port A1 in the asynchronous state(symbol SQ7), the ring apparatus 1 b of the standby system switches fromthe standby system to the active system. As described above, when theport A1 shuts down in a case where a fault is detected while a fault inthe control line Lf is detected, the ring apparatuses 1 a and 1 b switchbetween the active system and the standby system.

(Communication System in which the Ring Line Lr Between the RingApparatuses 1 a and 1 b is Duplexed)

In the above-described communication system, the ports A1 and B1 of thering apparatuses 1 a and 1 b are coupled by the control line Lf throughwhich only the control signal Sc flows. However, the communicationsystem is not limited to such a configuration. For example, the ports A1and B1 of the ring apparatuses 1 a and 1 b may be coupled by two ringlines through which both the main signal Sg and the control signal Scflow.

FIG. 14 is a configuration diagram illustrating an example of thecommunication system in which the ring line Lr between the ringapparatuses 1 a and 1 b is duplexed. Elements common to FIGS. 1 and 14are designated by the same reference symbols and will not be redundantlydescribed. The present example deals with a configuration where onlyring lines Lf1 and Lf2 between one pair of ring apparatuses 1 a and 1 bare duplexed. However, the configuration is not limited to suchduplexing. The other pair of ring apparatuses 1 c and 1 d may also beduplexed in similar manner.

The ring apparatus 1 a includes ports A1 x, A2, A3 x, and A4, and thering apparatus 1 b includes ports B1 x, B2, B3 x, and B4. The ports A3 xand B3 x are intercoupled so as to oppose each other through the ringline Lf1. The ports A1 x and B1 x are intercoupled so as to oppose eachother through the ring line Lf2. The ring line Lf1 is an example of acontrol line.

The main signal Sg and the control signal Sc both flow through the ringlines Lf1 and Lf2 while they are not faulty. The ports A3 x and B3 xmutually transmit and receive the main signal Sg and the control signalSc through the ring line Lf1. The ports A1 x and B1 x mutually transmitand receive the main signal Sg and the control signal Sc through thering line Lf2.

Consequently, the path R of the main signal Sg between the ringapparatuses 1 a and 1 b runs through the two ring lines Lf1 and Lf2. Theports A3 x and B3 x and the ports A1 x and B1 x are synchronizedrespectively by the control signal Sc transmitted and received throughthe ring lines Lf and L12.

FIG. 15 is a diagram illustrating an exemplary operation that isperformed by the communication system when a fault occurs in one ringline Lf1. Elements common to FIGS. 14 and 15 are designated by the samereference symbols and will not be redundantly described.

Upon detecting a fault in one ring line Lf1, the ring apparatuses 1 aand 1 b stops the transmission and reception of the control signal Scthrough the other ring line Lf2. Therefore, the bandwidth of the mainsignal Sg flowing through the ring line Lf2 may be increased by anamount equivalent to the bandwidth of the control signal Sc.

Although not illustrated, when a fault in the ring line Lf2 is detectedconversely to the present example, the transmission and reception of thecontrol signal Sc through the ring line Lf1 comes to a stop. Therefore,the bandwidth of the main signal Sg flowing through the ring line Lf1may be increased by an amount equivalent to the bandwidth of the controlsignal Sc.

When a fault is detected in only one of the ring lines Lf1 and Lf2, thering apparatuses 1 a and 1 b maintain the access lines La and Lb as anactive line and a standby line, respectively. This eliminates thenecessity of performing a switching process on the access lines La andLb.

FIG. 16 is a configuration diagram illustrating another example of thering apparatus 1 a. Elements common to FIGS. 2 and 16 are designated bythe same reference symbols and will not be redundantly described. Thering apparatus 1 b includes the ports B1 x, B2, B3 x, and B4 instead ofthe ports A1 x, A2, A3 x, and A4. However, the other elements of thering apparatus 1 b are similar to those of the ring apparatus 1 a.

The port A1 x transmits and receives the main signal Sg and the controlsignal Sc to and from the ring apparatus 1 b through the ring line Lf2.The port A3 x transmits and receives the main signal Sg and the controlsignal Sc to and from the ring apparatus 1 b through the ring line Lf1.

Upon reading a program from the ROM 11, the CPU 10 forms, as functions,fault detection sections 105 a and 106 a in place of the fault detectionsections 105 and 106, and a state control section 100 a, an MCLAGcontrol section 102 a, and an ERP control section 103 a in place of thestate control section 100, the MCLAG control section 102, and the ERPcontrol section 103, respectively. The fault detection section 105 adetects a fault in the ring line Lf1, and the fault detection section106 a detects a fault in the ring line Lf2.

The ERP control section 103 a controls the communication of the mainsignal Sg through the ring lines Lf1 and Lf2 in accordance with theresults of detection by the fault detection sections 105 a and 106 a andthe defect detection section 101. The ERP control section 103 a outputsthe results of detection by the fault detection sections 105 a and 106 ato the MCLAG control section 102 a.

The MCLAG control section 102 a controls the communication of thecontrol signal Sc in accordance with the results of detection by thefault detection sections 104, 105 a, and 106 a. As is the case with theMCLAG control section 102 in the foregoing example, the MCLAG controlsection 102 a switches the access lines La and Lb to an active line or astandby line.

The state control section 100 a is an example of the control section. Inaddition to the function of the state control section 100 in theforegoing example, the state control section 100 a coordinates with theMCLAG control section 102 a and the ERP control section 103 a to controlthe bandwidths of the main signal Sg and control signal Sc, which are tobe transmitted and received through the ports A1 x and A3 x. The statecontrol section 100 a causes the port A3 x to stop the transmission andreception of the control signal Sc through the ring line Lf1 inaccordance with the detection of a fault in the ring line Lf2 by thefault detection section 106 a, and causes the port A1 x to stop thetransmission and reception of the control signal Sc through the ringline Lf2 in accordance with the detection of a fault in the ring lineLf1 by the fault detection section 105 a. In this instance, the statecontrol section 100 a instructs, for example, the switch device 14 andthe MCLAG control section 102 a to stop the transfer and generation ofthe control signal Sc.

In accordance with the detection of a fault in the ring line Lf2 by thefault detection section 106 a, the state control section 100 a increasesthe bandwidth of the main signal Sg to be transmitted and received bythe port A3 x through the ring line Lf1. In accordance with thedetection of a fault in the ring line Lf1 by the fault detection section105 a, the state control section 100 a increases the bandwidth of themain signal Sg to be transmitted and received by the port A3 x throughthe ring line Lf1. The state control section 100 a instructs, forexample, the switch device 14 and the ERP control section 103 a toincrease the transfer bandwidth of the main signal Sg.

As described above, when a fault is detected in one of the ring linesLf1 and Lf2, the state control section 100 a is able to increase thebandwidth allocatable to the main signal Sg by stopping the transmissionand reception of the control signal Sc through the other ring line Lf1or Lf2. This suppresses a decrease in the main signal Sg between thering apparatuses 1 a and 1 b.

FIG. 17 is a flowchart illustrating an example of a control processperformed on the main signal Sg and the control signal Sc. The statecontrol section 100 a repeatedly performs the control process inparallel with the processes illustrated in FIGS. 9 to 11.

The state control section 100 a determines whether or not a fault isdetected in either the ring line Lf1 or the ring line Lf2 (step St41).If neither the ring line Lf1 nor the ring line Lf2 is detected to befaulty (“NO” at step St41), the state control section 100 a terminatesthe control process.

Meanwhile, if either the ring line Lf1 or the ring line Lf2 is detectedto be faulty (“YES” at step St41), the state control section 100 acauses the associated port A1 x or A3 x to stop the transmission andreception of the control signal Sc through the ring line Lf1 or Lf2 thatis not detected to be faulty (step St42). Next, the state controlsection 100 a increases the bandwidth of the main signal Sg transmittedand received through the ring line Lf1 or Lf2 that is not detected to befaulty (step St43), and then terminates the control process. In theabove-described manner, the control process is performed on the mainsignal Sg and the control signal Sc.

FIG. 18 is a state transition diagram illustrating exemplary states ofthe ring apparatuses 1 a and 1 b managed by the state control section100 a. State transitions common to FIGS. 8 and 18 will not beredundantly described.

In contrast to the state control section 100 in the foregoing example,the state control section 100 a transitions the ring lines Lf1 and Lf2from the normal state to the asynchronous state in accordance with theoccurrence of a fault in either the ring line Lf1 or the ring line Lf2(see “RING LINE FAULT OCCURRENCE”). In accordance with the faultrecovery of either the ring line Lf1 or the ring line Lf2 (see “RINGLINE FAULT RECOVERY”), the state control section 100 a transitions thering lines Lf1 and Lf2 from the asynchronous state to the normal state.

In the access line fault state, the state control section 100 atransitions the ring apparatuses 1 a and 1 b into the asynchronous statein accordance with the occurrence of a fault in either the ring line Lf1or the ring line Lf2 (see “RING LINE FAULT OCCURRENCE”) and with thefault recovery of the access lines La to Ld (see “ACCESS LINE FAULTRECOVERY”).

As described above, the state control section 100 a differs from thestate control section 100 in the foregoing example in conditions fortransitioning from the normal state or access line fault state to theasynchronous state and in conditions for transitioning from theasynchronous state to the normal state. However, the state controlsection 100 a is similar to the state control section 100 in the controlprocess performed in each state.

For example, in the asynchronous state, the state control section 100 ashuts down the port A1 x or the port A3 x, whichever is not detected tobe faulty, in accordance with the detection of a fault in the accessline La, as is the case with the operation illustrated in FIG. 6.Therefore, as is the case with the foregoing example, the ring apparatus1 a is able to permit the redundant partner ring apparatus 1 b tomaintain communication.

In the present example, the duplexed ring lines Lf1 and Lf2 are providedbetween the ring apparatuses 1 a and 1 b. However, the configuration isnot limited to such duplexing. For example, a part of the bandwidth ofthe control line Lf in the foregoing example may be used to conduct themain signal Sg. In such an instance, the MCLAG control section 102 maycoordinate with the ERP control section 103 to allocate the bandwidth ofthe main signal Sg.

The above-described embodiment is a preferred embodiment of the presenttechnology. However, the present technology is not limited to theabove-described embodiment. It may be understood that variousmodifications may be made without departing from the spirit of thepresent technology.

All examples and conditional language provided herein are intended forthe pedagogical purposes of aiding the reader in understanding theinvention and the concepts contributed by the inventor to further theart, and are not to be construed as limitations to such specificallyrecited examples and conditions, nor does the organization of suchexamples in the specification relate to a showing of the superiority andinferiority of the invention. Although one or more embodiments of thepresent invention have been described in detail, it should be understoodthat the various changes, substitutions, and alterations could be madehereto without departing from the spirit and scope of the invention.

What is claimed is:
 1. A communication apparatus comprising: a firstport that is coupled to a first communication apparatus through a ringline; a second port that is coupled to a second communication apparatusthrough a second access line, the second communication apparatus beingdisposed external to the ring line, the second access line beingconfigured redundantly with a first access line between the firstcommunication apparatus and the second communication apparatus; a thirdport that is coupled to the first communication apparatus through acontrol line for transmitting and receiving a control signal concerningthe first access line; and circuitry that detects a fault in the controlline, wherein the circuitry detects a fault in the second access line,switches the second access line from an active line to a standby line inaccordance with the detection of the fault in the second access line,and while the fault in the control line is detected, shuts down thefirst port in accordance with the detection of the fault in the secondaccess line.
 2. The communication apparatus according to claim 1,wherein, when no fault is detected in the control line, the circuitrytransmits the control signal for switching the first access line from astandby line to an active line to the first communication apparatusthrough the control line in accordance with the detection of a fault inthe second access line.
 3. The communication apparatus according toclaim 1, wherein the circuitry detects a defect in the communicationapparatus, switches the second access line from an active line to astandby line in accordance with the detection of the defect or a faultin the second access line, and individually shuts down the first port,the second port, and the third port in accordance with the detection ofthe defect.
 4. The communication apparatus according to claim 3, furthercomprising: a fourth port that is coupled to a third communicationapparatus through the ring line, wherein, when no fault exists in atleast either one of a section between the communication apparatus andthe first communication apparatus and a section between thecommunication apparatus and the third communication apparatus in a casewhere neither the defect nor a fault in the second access line isdetected, the circuitry maintains the second access line as an activeline.
 5. The communication apparatus according to claim 1, wherein thecircuitry detects a fault in the ring line, the first port transmits andreceives a main signal and the control signal to and from the firstcommunication apparatus through the ring line, the third port transmitsand receives the main signal and the control signal to and from thefirst communication apparatus through the control line, and thecircuitry causes the third port to stop the transmission and receptionof the control signal in accordance with the detection of the fault inthe ring line, and shuts down the third port in accordance with thedetection of a fault in the second access line while the fault in thering line is detected, and causes the first port to stop thetransmission and reception of the control signal through the ring linein accordance with the detection of a fault in the control line, andshuts down the first port in accordance with the detection of the faultin the second access line while the fault in the control line isdetected.
 6. A communication system comprising: a pair of communicationapparatuses coupled to a ring line, wherein one of the pair ofcommunication apparatuses includes a first port that is coupled to another one of the pair of communication apparatuses through the ringline, a second port that is coupled to an external communicationapparatus through a second access line, the external communicationapparatus being disposed external to the ring line, the second accessline being configured redundantly with a first access line between theexternal communication apparatus and the other one of the pair ofcommunication apparatuses, a third port that is coupled to the other oneof the pair of communication apparatuses through a control line fortransmitting and receiving a control signal concerning the first accessline, and first circuitry that detects a fault in the control line,detects a fault in the second access line, switches the second accessline from an active line to a standby line in accordance with thedetection of the fault in the second access line, and while the fault inthe control line is detected, shuts down the first port in accordancewith the detection of the fault in the second access line, and the otherone of the pair of communication apparatuses includes an access portthat is coupled to the external communication apparatus through thefirst access line, and second circuitry that detects a fault in thefirst access line, detects the shutdown of the first port, and while afault in the control line is detected and no fault in the first accessline is detected, switches the first access line from a standby line toan active line in accordance with the detection of the shutdown of thefirst port.
 7. The communication system according to claim 6, wherein,when no fault is detected in the control line, the first circuitrytransmits the control signal for switching the first access line from astandby line to an active line to the other one of the pair ofcommunication apparatuses through the control line in accordance withthe detection of a fault in the second access line.
 8. The communicationsystem according to claim 6, wherein the first circuitry causes one ofthe pair of communication apparatuses to detect a defect in the one ofthe pair of communication apparatuses, causes a first switching sectionto switch the second access line from an active line to a standby linein accordance with the detection of the defect or a fault in the secondaccess line, and individually shuts down the first port, the secondport, and the third port in accordance with the detection of the defect.9. The communication system according to claim 8, wherein the one of thepair of communication apparatuses includes a fourth port that is coupledto another communication apparatus through the ring line, and the firstcircuitry maintains the second access line as an active line when nofault exists in at least either one of a section between the one of thepair of communication apparatuses and the other one of the pair ofcommunication apparatuses and a section between the one of the pair ofcommunication apparatuses and the other communication apparatus in acase where neither the defect nor a fault in the second access line isdetected.
 10. The communication system according to claim 6, wherein thefirst circuitry detects a fault in the ring line, the first porttransmits and receives a main signal and the control signal to and fromthe other one of the pair of communication apparatuses through the ringline, the third port transmits and receives the main signal and thecontrol signal to and from the other one of the pair of communicationapparatuses through the control line, and the first circuitry causes thethird port to stop the transmission and reception of the control signalin accordance with the detection of the fault in the ring line, andshuts down the third port in accordance with the detection of a fault inthe second access line while the fault in the ring line is detected, andcauses the first port to stop the transmission and reception of thecontrol signal through the ring line in accordance with the detection ofa fault in the control line, and shuts down the first port in accordancewith the detection of the fault in the second access line while thefault in the control line is detected.
 11. A communication controlmethod that uses a communication apparatus including a first port, asecond port, and a third port, the first port being coupled to a firstcommunication apparatus through a ring line, the second port beingcoupled to a second communication apparatus through a second accessline, the second communication apparatus being disposed external to thering line, the second access line being configured redundantly with afirst access line between the first communication apparatus and thesecond communication apparatus, the third port being coupled to thefirst communication apparatus through a control line for transmittingand receiving a control signal concerning the first access line, thecommunication control method comprising: detecting a fault in thecontrol line; detecting a fault in the second access line; switching thesecond access line from an active line to a standby line in accordancewith the detection of the fault in the second access line; and while thefault in the control line is detected, shutting down the first port inaccordance with the detection of the fault in the second access line.12. The communication control method according to claim 11, wherein,when no fault is detected in the control line, the control signal forswitching the first access line from a standby line to an active line istransmitted to the first communication apparatus through the controlline in accordance with the detection of a fault in the second accessline.
 13. The communication control method according to claim 11,wherein the communication apparatus detects a defect in thecommunication apparatus, the second access line is switched from anactive line to a standby line in accordance with the detection of thedefect or a fault in the second access line, and the first port, thesecond port, and the third port are individually shut down in accordancewith the detection of the defect.
 14. The communication control methodaccording to claim 13, wherein the communication apparatus includes afourth port that is coupled to a third communication apparatus throughthe ring line, and the second access line is maintained as an activeline when no fault exists in at least either one of a section betweenthe communication apparatus and the first communication apparatus and asection between the communication apparatus and the third communicationapparatus in a case where neither the defect nor a fault in the secondaccess line is detected.
 15. The communication control method accordingto claim 11, further comprising: detecting a fault in the ring line;causing the first port to transmit and receive a main signal and thecontrol signal to and from the first communication apparatus through thering line; causing the third port to transmit and receive the mainsignal and the control signal to and from the first communicationapparatus through the control line; causing the third port to stop thetransmission and reception of the control signal in accordance with thedetection of the fault in the ring line, and while the fault in the ringline is detected, shutting down the third port in accordance with thedetection of a fault in the second access line; and causing the firstport to stop the transmission and reception of the control signalthrough the ring line in accordance with the detection of a fault in thecontrol line, and while the fault in the control line is detected,shutting down the first port in accordance with the detection of thefault in the second access line.